1. Technical Field
The present invention relates to a method for laminating a substrate in which plural substrates made of materials such as glass material, semiconductor material, dielectric material, metallic material, ceramic material and the like are laminated with each other so as to obtain a directly joined substrate having very small warping and distortion, and a laminated substrate and a directly joined substrate obtained by means of the method.
2. Related Art
A direct joining technique is known for firmly joining plural substrates made of various materials such as glass material, semiconductor material, ferroelectric material, metallic material, piezoelectric ceramics material and the like with each other with high accuracy without using an adhesive agent and the like, so as to realize a device having various characteristics. The direct joining technique for joining substrates made of different kinds of materials has particularly high usefulness.
Examples of a directly joined substrate obtained by laminating substrates made of different kinds of materials include a directly joined substrate using oxide ferroelectric crystals, directly joined substrates between glass and LiNbO3 crystal substrate (hereinafter, also referred to as a LN crystal substrate) or between glass and a LiTaO3 crystal substrate (hereinafter, also referred to as a LT crystal substrate), and the like. Another example of the material includes semiconductor substrates which are directly joined with each other so as to form a SOI (Silicon On Insulator) substrate, and this method is widely known.
For example, it is expected that a directly joined substrate between substrates made of different kinds of materials such as a glass material substrate, a dielectric material substrate, a metallic material substrate, a semiconductor material substrate and the like is applied to an optical device. In this application, one of the laminated substrates of the directly joined substrate is processed into a thin plate and then subjected to a ridge processing, so that the resultant can be utilized as an optical waveguide-type device.
In general, a functional device such as an optical device is manufactured by a process such as polishing and etching to a directly joined substrate. In the steps of polishing and etching, at the time of holding the directly joined substrate in the process unit, if the directly joined substrate has distortion or warping, the directly joined substrate is not held uniformly and the accuracy of the process is lowered. In particular, in the step of polishing into a thin plate requiring high accuracy and in the step of dry-etching requiring a large area, the magnitude of the distortion of the directly joined substrate has large influence on the characteristics of the device and the yield percentage of the product. Therefore, in order to obtain a functional device with high performance, it is required to reduce distortion and warping by increasing the accuracy of lamination of the directly joined substrate.
Here, conventional methods for manufacturing a directly joined substrate will be described.
For example, there is a method in which substrates located at positions close to each other are slightly pressurized to locally create a tightly attached state, and then, the vicinity of the tightly attached areas is further pressurized to expand the tightly attached areas, thereby allowing the substrates to be tightly attached to each other over their entire surfaces. However, the tightly attached portions are created unevenly if the substrates are distorted prior to attachment. The unevenness of the tightly attached portions results in distortion in the directly joined substrate and causes failed lamination.
A schematic diagram of the state where the distortion is created is shown in FIG. 7. In FIGS. 7A to 7C, the reference numeral 7 denotes a X-cut MgO-doped LiNbO3 crystal substrate (hereinafter, also referred to as a MgO:LN crystal substrate); 8, a X-cut LiNbO3 crystal substrate; and 9, pressurizing means. It is assumed that the surfaces of the MgO:LN crystal substrate 7 and the LN crystal substrate 8 to be joined to each other are optically polished beforehand. As is shown in FIG. 7A, if a part of the MgO:LN crystal substrate 7 is pressurized locally by the pressurizing means 9 in a state where the MgO:LN crystal substrate 7 and the LN crystal substrate 8 are held at positions close to each other, the local pressurization creates a tightly attached state with the substrates distorted. Specifically, since a stress arises from the center of pressurization outwardly on the joined surfaces immediately below the pressurized portions, the surfaces are tightly attached to each other with their crystal grating expanded. When a new portion in the vicinity of the tightly attached areas is pressurized, as shown in FIG. 7B, the new pressurized portion is subjected to a force spreading from the center of the pressurization outwardly, whereas the already tightly attached portion is subjected to stress that extends toward the attachment point, that is, in the direction where the crystal grating shrinks.
As is generally known, the substrates which are capable of directly joining with each other can be tightly attached with each other at high strength even without performing a heat treatment, and their attached portions never detach from each other even if a relatively large force is applied thereto. Therefore, the distortion remains on the attached portions. If the tightly attached state is created starting from the centers of the substrates to be laminated to each other and the attached area is expanded toward the ends of the substrate, as shown in FIG. 7C, a directly joined substrate which is seriously distorted when seen as a whole is formed.
There is a method proposed for reducing the distortion created at the time of lamination described above, and further for removing air bubbles and air voids which will cause faulty lamination and thus induce a decrease in the yield percentage during the mass production of functional devices.
For example, Japanese Unexamined Patent Publication No. 09-63912 describes a method in which escape grooves are formed on substrates so as to remove air bubbles and air voids from the surfaces to be laminated. The escape grooves also reduce the stress exerted at the time of pressurization. However, when two distorted substrates are brought into contact with each other, pressurizing a portion or the entire area the individual substrates is required in order to completely tightly attach the substrates to each other. In this case, although the number of local areas where the substrates are not joined to each other is reduced, large distortion in the directly joined substrate as a wholes apparent.
The situation where distortion is created in this case will be described based on FIG. 8. In FIGS. 8A to 8D, the reference numeral 10 denotes a X-cut MgO:LN crystal substrate; 11 a X-cut LN crystal substrate formed with escape grooves; 12, a pressurizing means; and 13, escape grooves. It is assumed that the surfaces of the MgO:LN crystal substrate 10 and the LN crystal substrate 11 to be laminated to each other are respectively optically polished beforehand. As shown in FIG. 8A, when the MgO:LN crystal substrate is locally pressurized by the pressurizing means 12 in a state where the MgO:LN crystal substrate 10 and the LN crystal substrate 11 are held at positions close to each other, a tightly attached state is created with the substrates distorted because the substrate is locally pressurized. At this time, the escape grooves 13 serve to allow the stress generated on the joined surface to escape. However, depending on pressurizing methods, as shown in FIG. 8B, the substrates are joined with each other at both ends of each escape groove 13 which has absorbed distortion. In this case, large distortion remains on the MgO:LN crystal substrate 10 at its escape grooves 13, which are areas where the substrates are not joined to each other. Further, as shown in FIG. 8C, when the substrates are pressurized at their portions immediately above the escape grooves 13, the substrates are joined with each other with the crystal grating expanded on the escape grooves 13. For this reason, when the pressure is released, as shown in FIG. 8D, the residual stress is exerted to the substrates from the escape grooves 13 outwardly. As a result, the resultant directly joined substrate warps largely.
On the other hand, Japanese Unexamined Patent Publication No. 07-283379 proposes a method in which two substrates are held at positions close to each other and are pressurized by compressed air so as to be laminated with each other. However, distortion is still created due to the pressurizing step as is the case in the foregoing technique, and it is difficult to remove the distortion on the directly joined substrate.
Further, Japanese Unexamined Patent Publication No. 2000-216365 proposes a method for laminating substrates with each other by their own weights under reduced pressure so as to avoid encapsulation of air into the laminated surfaces and reduce the distortion of the substrates. However, since spacers for holding the substrates locally support portions of the substrates, the held substrates originally have distortion due to their own weights. Therefore, the substrates are likely to distort when laminated with each other.
Further, Japanese Unexamined Patent Publication No. 06-267804 proposes, as a method for correcting the warping of a directly joined substrate that includes forming a thin film having a stress on the respective surfaces to be laminated. However, it is difficult to establish a coincidence between the distribution of the distortion of the substrate created in the film formation step and the distribution of the distortion of the substrates to be laminated. There is also a problem that it is required to observe distortion of each substrate in the laminating step, and then select and manage a thin film having stress capable of correcting the distortion. Further, the roughness accuracy on the surface of the film may be poor depending on the material of the thin film to be formed or the film formation method. Forming the direct joint becomes difficult when the roughness accuracy is poor.
Further, all of the methods described above have a problem that significant distortion appears when substrates made of a less rigid, easily deformable material or thin substrates having a thickness of 1 mm or less are laminated with each other. In particular, when crystal substrates made of oxide material are directly laminated with each other, since the crystal substrates have high rigidity, it is very difficult to correct the warping of the substrate by a method such as applying external force thereto after the substrate is obtained by directly joining substrates with warping remaining. Thus, there has been a demand for a lamination method for obtaining a substrate having small distortion and warping.